I know it is a crazy question. The standard party line answer will be whatever you want to spend, it's a free society. But yet there are guidelines to financial decisions. When you buy a house, the bank restricts you to certain percentages of your annual gross income if you need a mortgage.
There are also common sense guidelines. Someone living in a $200,000 house would not be driving a $350,000 Ferrari much less a $1 Million plus Bugatti.
So what percentage of net worth is appropriate for a single watch?
Let's say you are fortunate enough to be worth 1 million dollars. At 1%, this would put you on the $10,000 range: Rolex, nicer IWC, JLC, GO, etc. Up it to 3% and you get to play in the entry level Patek, Lange, Journe world. If you want to get really crazy, up it to 10% and you get upper end Lange, Patek chronograph/annual calendar, etc.
There are may tangents to this question. You could also apply this principle to your entire collection. What percentage of your net worth would you commit to your entire watch collection? We are all WIS's and want to own the best watches but we also have to have a minor degree of sensibility and meet our other responsibilities.
What say you?